Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a wastewater treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. Thus, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers or other waterways.
The overflows, called combined sewer overflows (CSOs), contain not only stormwater but also untreated human and industrial waste, toxic materials and debris. Overflows are a major source of water pollution in communities with combined sewer systems.
The problems related to overflow and infiltration of untreated water into sewer systems also occur in closed (non-CSO or separate) wastewater systems. Due to deteriorating collection systems and illegal cross-connections, discharge of untreated wastewater is a common occurrence in closed systems as well. Excess flows in either system can take the form of infiltration into the system due to cracked pipes, offset pipe joints, and the like, or direct inflow into the system, such as with direct surface water connections, foundation drains, and the like.
The costs of remedying the wet weather overflow problem are not insignificant. In Pennsylvania alone, it was estimated in 2001 that over $4 billion would be needed to bring wastewater discharge into compliance with state and national water quality standards. Estimates of the repair costs for wastewater and drinking water systems statewide in Pennsylvania exceed $11 billion dollars.
One method of managing wet weather overflows is to build overflow tanks positioned outside the wastewater treatment plant, before the wastewater point of entry into the plant. When the flow rate into the plant is heavy, wastewater is diverted into the overflow tank(s), and the water is stored in the tank until levels within the plant subside and the plant can accommodate the additional flow. A problem with this method is that overflow tanks are very expensive to build. For example, the cost of building a 700,000 gallon capacity tank in 2005 is about $3.1 million dollars. Additionally, if the capacity of the overflow tank is exceeded, wastewater is diverted to surface waterways, bypassing the treatment process and degrading the water quality of the waterway. Most methods of control aimed at increasing the capacity of the plant are very expensive and are land intensive.
There remains a need for a cost-effective solution to the problem of reducing and managing wet weather overflow in the wastewater treatment process so that water discharged to surface waters meets the applicable state and national water quality standards.